1. Field of the Invention
The present invention relates to a video signal reproducing apparatus such as a video disc player or a video tape recorder and, more particularly, to a time base correcting circuit designed for correction of time base fluctuation in a reproduced video signal.
2. Description of the Prior Art
In a video signal reproduced by a video disc player or a video tape recorder, there occurs a time base fluctuation called jitter that results from the mechanical precision of a recording medium, a rotary mechanism and so forth, hence causing some deterioration of the picture quality such as instability of the picture on a screen.
Furthermore, there also occurs in the video signal a dropout with absence of information due to foreign particles such as dust deposited on a recording medium or some other fault thereof, whereby the picture quality is deteriorated in this case as well.
In order to avert such a trouble, the video disc player or video tape recorder is generally equipped with a jitter correcting means and a dropout correcting means to process the reproduced video signal properly, thereby preventing the above-mentioned deterioration of the picture quality.
Although jitter correction may be executed by mechanically vibrating a signal detector through the use of an actuator, it is customary to adopt an electrical method employing an IC memory, as disclosed in U.S. Pat. No. 4,212,027.
As regards the means for dropout correction, there is known a method which first detects a dropout period by sensing the pulse duration of a reproduced signal (usually frequency-modulated signal) obtained from the signal detector or by sensing the reduction of the signal envelope, and then replaces the signal during such dropout period with a closely correlated signal preceding by one horizontal scanning interval.
In the above jitter correcting means of the prior art employing an IC memory, the reproduced video signal needs to be converted from an analog form to a digital one so as to be processed as a digital signal. Therefore, when using such jitter correcting means in combination with the dropout correcting means, a remarkable advantage is attainable with respect to both the circuit configuration and the performance if the replacement with a signal preceding by one horizontal scanning interval is executed in the form of a digital signal in the dropout correcting means.
As a result of studying the system that uses such two means in combination, the present inventors have found the following. Since jitter information required for jitter correction is detected from the video signal prior to dropout correction, there exists a problem that erroneous detection of the jitter information is prone to be executed to bring about malfunction of the jitter corrector, and such malfunction generates local skew on a screen to eventually cause considerable deterioration of the picture quality.
As the sampling in analog-to-digital signal conversion needs to be executed in the stage where the signal still has the jitter, the aforesaid jitter information is exactly required for such analog-to-digital conversion, so that it becomes necessary to detect the jitter information in a stage prior to analog-to-digital conversion. However, in case dropout correction is performed digitally as mentioned above, it follows that the dropout corrector is disposed in a stage following the analog-to-digital conversion, whereby detection of the jitter information should be executed inevitably from the video signal prior to dropout correction.